Adaptive optical system technology has found a wide range of applications including astronomical imaging and long-range free space optical communication. Adaptive optical system technology can potentially enhance any application in which turbulence occurs along the path, leading to refractive index fluctuations due to temperature variations. This turbulence degrades the performance of an imaging or laser projection system. Prior art methods (Ref. 1-13), as referenced in the references cited section below, have been developed to provide methods for compensation of the effects of turbulence on laser propagation through the atmosphere. One of the critical limitations of adaptive optical system technology is the signal level required to make measurements with sufficient accurate to effect high quality compensation for the projected laser beam. While some methods in particular (Ref. 9 and Ref. 10) minimize the required signal for wavefront sensing by use of heterodyne methods, the use of heterodyne methods introduces complexity in adaptive optical system that is not desirable for some applications.
What is needed is a method for wavefront sensing that requires minimum signal level and also minimizes hardware complexity. The present invention meets these needs by developing a variation of a relatively old technology, the classical Foucault knife edge test and the pyramid sensor (Ref. 14), and incorporating a means to compensate for inherent variations in the system gain associated with the transition across the knife edge. As with the pyramid sensor, when operated with a null-seeking adaptive optical system, the knife edge wavefront sensor converges to a state that minimizes signal requirements for the wavefront sensor as the compensation system reaches steady state and a partial level of compensation. The variations in the knife edge wavefront sensor gain with partial compensation of the received wavefront are compensated to take full advantage of the knife edge wavefront sensor gain. A known limitation of the knife edge principle is that the knife edge gain is reduced as aberrations increase. Thus, before the adaptive optical system is operating, the knife edge wavefront sensor gain is reduced, leading to a reduced effective bandwidth that must be compensated to enable the adaptive optical system to initially converge. However, as the adaptive optical system converges and improves the level of compensation, the knife edge wavefront sensor gain increases. If the increased knife edge wavefront sensor gain is not compensated in the system real time controller as the system converges, then the adaptive optical system will become unstable and will be incapable of providing compensation. Thus, gain calibration is required for effective operation of the knife edge wavefront sensor when incorporated in an adaptive optical system. The present invention meets this requirement by incorporating a means for real time gain calibration of the knife edge wavefront sensor.